Feedback needed on concrete work

gratefulgaryAugust 27, 2014

So I finally got this slab poured yesterday, by the guy who is my contractor friend. Let me preface my question(s) with the disclaimer that he had put an addition on my barn which included pouring a slab for the floor that turned out just fine.

He used rebar for this job. The slab was ~six in., ~35X30. The slab is in four "squares." As regarding the rebar, he laid it out in a grid which extended under the cross forms (not the perimeter). That is, each square did not have its own grid and the bar was laying on the ground. I brought to his attention that there wouldn't be much reinforcement that way and he assured me that they would lift it up during the pour so it would end up in the bulk of the concrete. Seeing as how the grid was on the ground under the cross forms I didn't really see how that was going to work so well. And in fact I don't think it did.

While pouring the second square they had too much concrete and so shoveled the excess into what turned out to be square four. In the mean time it was walked on, a lot, and by the time square four was poured it was pretty hard. Is that concrete now part of the slab, or is the slab thinner in those areas by the thickness of the hardened material?

In two of the squares they didn't do such a good job screeding and there are undulations. I mean, put a level across the top and there is an 1/2 - 3/4 in. space underneath. Is this something I should make an issue of? I really don't consider this to be acceptable but if it sounds like it's within some industry acceptable standard I can live with it. If it isn't, how can it be made reasonably flat?

He used pressure treated 2X4 for the cross frame, which means there is concrete beneath the wood. Is that an issue, like is that now a two inch slab with a piece of wood on top of it? The wood was left in place as the expansion joints. Also, the wood sits proud of the concrete surface for some lengths depending on how well they screed. Should I ask that it be knocked down? It looks to me that it's going to be a problem the first time I plow this winter.

This slab has to support the weight of tri-axle loads of logs driving over it a few times a year.

I'd like to hear some comments, mainly constructive in nature if possible.



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Sophie Wheeler

Without standoffs, and a whole network of wired rebar, log truck traffic will probably crack it into rubble after a few trips after a winter.

    Bookmark   August 28, 2014 at 7:26AM
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I don't understand why some concrete contractors still do the "pull up the rebar" method. Using chairs is not that expensive (but it's more difficult to walk across the area).

Pulling up rebar is placing the rebar at a non-controlled height within the slab. If it is too high, it won't work because it won't pick up tensile forces on the bottom of the slab and if it is not lifted up, it may as well be not there at all.

If the rebar is not placed correctly, your slab will crumble over time. I have seen this many times and always think it is sad that all the work and expense of placing concrete is ruined by sloppy rebar work.

    Bookmark   August 28, 2014 at 3:08PM
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The slab should have been speced to the heaviest weight it had to carry. If not, even correctly supported rebar on "chairs" wouldn't help. Exterior self-levelling concrete can be added to your new slab by experienced professionals.

This post was edited by worthy on Thu, Aug 28, 14 at 15:59

    Bookmark   August 28, 2014 at 3:58PM
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6"' concrete should be really durable even with the faults you outlined.

    Bookmark   August 28, 2014 at 9:47PM
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Rebar should not be lifted into place; it should be installed on chairs and tied together with wire ties. I don't even see how it would be possible to lift even #4 bars after they had been tied.

Perhaps you are talking about Welded Wire Fabric which is light enough to be lifted to the center of the slab during the pour. WWF is more for shrinkage cracking prevention than strength.

Placing excess concrete from one pour into adjacent forms is a sign of incompetence. It should be placed elsewhere and a bent bar installed to allow it to be moved later.

I suspect the contractor had not built a structural slab this large before and simply made a mess of it. This is not typical residential construction so you needed a more experienced contractor.

    Bookmark   August 28, 2014 at 10:31PM
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Thanks for the responses. It wasn't mesh, it was rebar.

    Bookmark   August 30, 2014 at 7:09AM
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I posted photos here:

My friend further states that expansion joints accommodate expansion/contraction through the thickness, not horizontally. Is that true?

His solution is to remove the wood and replace with appropriate expansion joint material, cut the slabs loose from one another and grind out the surface discontinuities with one of those grinders that looks like a floor polisher. And provide a five year warranty that if it fails he'll then remove the slabs and redo the job correctly.

    Bookmark   September 3, 2014 at 10:22AM
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Wow, that's not pretty. The woods going to rot out over time which is going to leave big gaps. Can't quite figure out how they got it so wavy. I thought my concrete guys were cutting corners a bit but they did a way better job than that.

    Bookmark   September 3, 2014 at 11:12AM
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There are expansion joint materials that would leave a neater joint.

    Bookmark   September 3, 2014 at 7:19PM
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